1. Field of the Invention
This invention relates to the preparation and uses of a storage-stable, moisture-cured, single-component polyurethane system which does not exhibit the disadvantages known to the single-component polyurethane systems of the prior art. This invention also relates to simplifying processing, shortening curing time, and improving the mechanical properties of the resulting final products, thereby facilitating new areas of application for polyurethane systems made according to this invention.
Surprisingly, it has been discovered that by substituting aliphatic polyisocyanates with reactive aromatic polyisocyanates in the preparation of the polyurethane prepolymers, the storage stability of the single-component polyurethane systems was maintained while both curing and the mechanical properties from the resulting polyurethanes were substantially improved. By using non-aromatic polyamines in forming polyaldimines, preparation is simplified, since the reaction water does not have to be separated by a special water separation process. Rather, the water separates off without processing. Moreover, the spectrum of mechanical properties in the final products is expanded, thereby allowing new applications. The compounds which are used as blocking agents in the preparation of polyaldimines have no reactive hydrogen atoms in the .alpha.-position relative to the carbonyl groups. Thus, direct reactions between polyaldimines and polyisocyanates as described in the Journal of Organic Chemistry 33, (1968), pages 2357-2361 by J. P. Chupp and E. R. Weiss, are not possible, so that storage-stable mixtures of polyaldimines and organic polyisocyanates result.
Two-component polyurethane systems generally find application as coating agents and are also useful for the preparation of cushioned flooring, such as athletic field flooring. However, these systems are subject to several disadvantages. One disadvantage is that the formulations do not have a long pot life, whereby frequently one must use organic mercury compounds as catalysts. Furthermore, it is generally not possible to process the systems at temperatures below 10.degree. C. because of their poor curing. Since carbon dioxide forms through the reaction of the polyisocyanate components with water, and the formation of bubbles may also occur, the dual component polyurethane systems may be processed only under dry weather conditions outdoors. Another disadvantage is that one must work under difficult conditions on construction sites with complicated and expensive equipment. Accordingly, there may be insufficient safety precautions when processing on site.
2. Description of the Material Art
In order to overcome these and other disadvantages, moisture-cured, single-component polyurethane systems have been formulated which comprise isocyanate prepolymers as starting components. U.S. Pat. No. 4,025,579 (German patent No. 24 47 625) discloses isocyanate prepolymers based on hydroxyl group-containing polyethers and polyisocyanates having a 2,4'-methylene-bis-(phenylisocyanate) content of from about 20 to 80 weight percent. The polyethers and polyisocyanates are useful as binders for rubber and/or elastic plastic granulated materials for the preparation of elastic coatings. However, the mechanical properties, especially the tensile strength at break for such coating materials, is deficient for many areas of application.
U.S. Pat. No. 4,481,345 (German patent No. 33 08 418) discloses formulations containing organic polyisocyanates and ketimine-blocked polyamines containing amino groups which are useful for improving the mechanical properties of polyurethane systems. However, the pot life of coating or sealing compounds in a maximum of 17 days. Accordingly, the products made according to this patent are not storage-stable, single-component polyurethane systems.
U.S. Pat. No. 3,743,626 (German patent No. 20 18 233) discloses hydrolysis-sensitive heterocycles which are used as capped chain extending agents in single-component polyurethane systems. The mixtures are curable with water and are based on oxazolidines and organic polyisocyanates. The storage time of systems which are based on reactive aromatic polyisocyanates is very short, even if moisture is excluded. Further, systems based on aliphatic polyisocyanates cure too slowly in the presence of moisture.
U.S. Pat. No. 4,118,376 (German patent No. 26 32 513) discloses prepolymers comprising isocyanate groups, instead of organic polyisocyanates, from organic polyisocyanates and dispersions of polymers, polycondensation or addition polymerization products, used in polyhydroxyl compounds. However, these products have relatively low mechanical strength. Because of the chemical structure of the chain extending agents, no rigid phase aggregation or rigid phase crystallization is possible and the elastomers disclosed in U.S. Pat. No. 4,118,376 possess a strain modulus of only 0.2 N/mm.sup.2 at 100 percent elongation.
U.S. Pat. No. 4,469,831 (German patent No. 33 06 373) discloses moisture-curing, single-component polyurethane systems which comprise prepolymers having an NCO content of from 1 to 5 weight percent based on aliphatic and/or cyloaliphatic polyisocyanates and dialdimines. The prepolymers are prepared by the reaction of aliphatic, cycloaliphatic and/or aromatic diamines with 3-(isopropyl-carbonyloxy)-2,2-dimethylpropanal. The polymers so formed have a storage stability of more than 1 year. Yet, even these products do not fulfill all of the requirements with regard to mechanical properties of the cured polyurethane elastomers and with regard to the curing time. In addition, the polyurethane system of U.S. Pat. No. 4,469,831 is subject to sporadically annoying odors.
German patent No. 36 07 996 discloses moisture-curing, storage-stable, single-component polyurethane systems which comprise a polyurethane prepolymer having an NCO content of from 1 to 10 weight percent, prepared by the reaction of higher molecular polyols and aromatic polyisocyanates and polyaldimines based on arylene radicals.